To prepare sufficiently soggy spaghetti atop the summit of Mt. Everest, you might want to bring salt in addition to a stove. Why not sugar? In addition to tase bud considerations (unless you’re Buddy the Elf…) sugar (as a non-electrolyte) has less BOILING POINT ELEVATION power.

Yesterday, I told you about how autoclaves use high pressure to raise the boiling point of water so thats the steam pumped into their oven-like chambers gets really really hot – hot enough to kill any microbes living on things you want to sterilize. This is similar to how pressure cookers work – raise the pressure so that you can get things hotter without your food evaporating to nothingness. Pressure is just one way to manipulate boiling point, and it reflects a deeper point – boiling point is NOT simply an inherent property in a substance – instead it depends on external circumstances – like pressure & what else is around (solute concentration). So, as I’m busy experimenting today, I thought it’d be a good time to review. 

VAPORIZATION is going from a LIQUID to a GAS. It happens when liquid molecules have enough energy to vibrate enough to break free from the attractive INTERMOLECULAR FORCES (IMFs) “gluing” them to neighboring molecules. BUT in order to REMAIN a gas, a molecule also has to “push away” surrounding particles so it has enough space to move around & avoid getting pulled back in (CONDENSING (GAS to LIQUID)) 

The gas’ “pushing power” can be described as its VAPOR PRESSURE & the more gas molecules pushing, the higher the vapor pressure. At & above a temperature called the BOILING POINT (b.p.), the vapor pressure’s strong enough to push away whatever external pressure there is & remain a gas.

Visualization time! Imagine a closed system, where molecules are vaporizing from a surface of a liquid, but they can only go a little ways before they’re blocked by a lid. The evaporated molecules push against the lid & if there were enough gas molecules, they’d produce sufficient VAPOR PRESSURE to push through that lid & escape.  BUT the lid’s too strong bc molecules in a SOLID are held together tightly & don’t want to budge.

BUT what if that lid were easier to get through?  What if, instead of being solid, it was made of GAS? In a gas, molecules are more separated & easier to get through, so our escaping molecules might stand a chance. At the SURFACE of a liquid, this “lid” is the ATMOSPHERIC PRESSURE (the weight of the air pushing down) 

It’s easier to visualize this occurring at the surface, but if you add enough energy (which you can do by adding heat, which is a form of energy), vaporization happens away from the surface & the vaporized molecules travel to the surface as pockets of gas molecules – BUBBLES! The bubbles have their own liquid-gas interfaces so they’re like mini versions of what’s going on at the top surface, but here it’s gas inside of the bubble creating vapor pressure that pushes away surrounding liquid 

BUT the “lid” on the bubbles is stronger because they have additional forces to combat – HYDROSTATIC PRESSURE (the water above weighing down on it) as well as SURFACE TENSION (liquid lining the bubble trying to “cinch” it in)  so it’s harder for them to escape.

At the b.p. the vapor pressure’s strong enough to counterbalance these forces so the bubble doesn’t collapse. INSTEAD the bubble grows bigger as more molecules join it & it pushes through to the surface where it can escape.


You might see b.p. defined as temp at which vapor pressure = atmospheric pressure, but the external pressure doesn’t have to be the atmospheric pressure.

Changing either the vapor pressure or the external pressure will affect the “boilablity” 👉something will boil more readily if the vapor pressure is higher or the external pressure is lower 

You can lower EXTERNAL PRESSURE by applying a vacuum & “sucking away” some of the above particles OR you can “just” climb Mt. Everest! At high elevations, there’s less air above pushing down, so atmospheric pressure’s lower.

You can increase VAPOR PRESSURE by adding heat, so more molecules have a chance of having enough energy to convert to a gas

You can decrease VAPOR PRESSURE by adding SOLUTE (dissolving stuff in the liquid (which we call the SOLVENT)). No matter what* you dissolve, the vapor pressure will decrease (Raoult’s law). It’s one of those COLLIGATIVE PROPERTIES where it’s only the # of dissolved particles that matters *exceptions apply:

Adding solute means there’s fewer solvent molecules at liquid-gas interfaces (both at top surface & around the bubbles) so it’s harder to “recruit” converts. And because the solutes get in the way of some of the solvent-solvent interactions, the solvent molecules are “freer” (have greater ENTROPY) even though they’re still in the liquid phase, so they’re more content

So fewer molecules are converting from liquid to gas. So there are fewer gas molecules there to exert vapor pressure and we call this VAPOR PRESSURE DEPRESSION

In order to get sufficient molecules to vaporize so vapor pressure’s high enough to match external pressure you have to add more heat. So even if solute makes a solvent molecule less likely to be in a position to do so (be at a liquid-gas interface) the ones that are are more likely to be able to vaporize. So vapor pressure depression leads directly to another colligative property, BOILING POINT ELEVATION

Since these depend on the number of dissolved particles, ELECTROLYTES like table salt (NaCl,) which dissociate into more particles when you dissolve them have a stronger effect than NON-ELECTROLYTES like table sugar (sucrose) which don’t

Combining these concepts & going back to our stiff spaghetti scenario… atop Mt. Everest  atmospheric pressure is lower, so external pressure is lower, so vapor pressure doesn’t have to get as high to match it, so water in your pot boils at lower temperatures. BUT even though water’s boiling your spaghetti might not be hot enough. You can ADD SALT to lower VAPOR PRESSURE & raise b.p to counterbalance this. Alternatively, you could raise b.p. by raising EXTERNAL PRESSURE, which is the theory behind a pressure cooker! (and the autoclave)

TO SUMMARIZE: Boiling can occur when VAPOR PRESSURE = EXTERNAL PRESSURE. So whether something will boil depends on

  • EXTERNAL PRESSURE – lower pressure pressure makes it easier to boil (you don’t need as much heat) 
    • depends on elevation, depth in liquid, whether you’re in a vacuum, a pressure cooker, etc.
  • VAPOR PRESSURE  – more VAPORIZATION means higher vapor pressure, which lets it match & exceed the external pressure. It depends on:
    • TEMPERATURE – at higher temperatures  molecules have more energy, so more molecules convert to gas state
    • NATURE of IMFs – stronger bonds require more energy to break, so you have to add more heat to get it to boil 
    • AMOUNT OF SOLUTE – more dissolved particles lowers solvent molecules in position to vaporize & solvent molecules have more entropy so there’s less benefit to vaporize (they’re more content) – so you get lower vapor pressure

Leave a Reply

Your email address will not be published.